The present invention relates to a communication method which is adapted for transmission and reception between communication apparatuses connected by a 2-wire system transmission line.
For the remote control of valves in industrial processes, for example, a receiving system called a positioner is generally provided. A signal is sent to this positioner from a central control unit as a current value which changes in the range of 4-20 mA, for example. The receiving apparatus receives this signal and controls devices in accordance with a current value.
However, in this case, the control signal is sent only by way of an analog current value and, therefore, it is impossible to send simultaneously two kinds of data, and when it is required to send a monitor signal to the control unit for control purposes, another transmission line and transmission apparatus must be provided additionally. Moreover, two pairs of transmission lines and transmission apparatus for each direction of transmission are required in case two kinds of data are respectively transmitted or received simultaneously by the control units.
For simultaneous transmission and reception of two kinds of data, the prior art has the disadvantages that the facility cost increases, maintenance and inspection are required for more areas, and the man-hours for preventative work also increases.
The present invention has an object to essentially solve such problems of the prior art and simultaneously provides a very effective communication method which is capable of simultaneously transmitting and receiving two kinds of data.